feat: strip alpha channel in vips image processor

[knarewski]

Background

GdkPixbuf-based image processing can take over 1GB of memory for processing a single high-resolution photo. We've decided to introduce another processor, hoping that it'll improve Morandi's memory profile and performance.

Problems

By default vips handles transparency differently to pixbuf when saving to jpeg.

• GdkPixbuf discards alpha channel (ie treats all pixels as opaque)
• Vips flattens the image with black background (ie composes image with black, adjusting RGB values to account for transparency)

Example:

require 'morandi'
GdkPixbuf::Pixbuf.new(file: 'spec/fixtures/match-with-transparency.png').save('tmp/pixbuf_output.jpg', 'jpeg', quality: '97')
Vips::Image.new_from_file('spec/fixtures/match-with-transparency.png').write_to_file('tmp/vips_output.jpg', Q: 97)

Comparison of transparent images saved using both libraries (left - pixbuf, right - vips)

Both processors have some exceptions, where transparency is handled inconsistently, eg:

• When cropping outside the images boundaries, both processors flatten the transparent image against a white background
• When straightening, Pixbuf-based processor flattens the image

Solution

Discard alpha when processing with Vips to increase the consistency when saving to jpeg.

Decision

• It feels that flattening alpha on white would be the closest to the expected outcome, but I decided to discard it ATM to better match current GdkPixbuf's behaviour when saving to jpeg
• Saving to PNG via GdkPixbuf retains alpha; since that operation is not part of Morandi.process's public interface and VipsProcessor doesn't expose writing to png at all, I ignored consistency in that area for now
• Stripping alpha as a last step gives two benefits: 1. the quality of processing should be slightly better (as RGBA holds more information than RGB), 2. the benchmarks are more relevant, as they account for processing all the channels

Pixbuf vs Vips comparison

Processing a transparent image with gamma+sepia+crop (left - pixbuf, right - vips), mean average error: 0.000500558 - that's how transparency is handled by GdkPixbuf-based processor for most operations

Processing a transparent image with gamma+sepia+crop+straighten (left - pixbuf, right - vips), mean average error: 0.0422601 the incosistent pixbuf processor's behaviour; transparency is deliberately stripped in Vips to increase consistency